Embodiments of the present specification relate generally to a neutral point clamped converter, and more specifically to a shutdown method for the active neutral point clamped converter.
Typically, three-level active neutral point clamped (3L-ANPC) converters are employed to convert a direct current (DC) power to an alternating current (AC) power having a three-level output. The 3L-ANPC converters are widely used in AC drives and flexible AC transmission systems. These 3L-ANPC converters employ a plurality of switches to aid in the power conversion. Also, traditionally, 3L-ANPC converters that employ silicon carbide (SiC) switches such as SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) are used when it is desirable to withstand a high voltage.
As will be appreciated, SiC MOSFETs switch faster in comparison to other switches such as insulated gate bipolar transistors (IGBTs), thereby resulting in lower switching losses. However, the faster switching of the SiC MOSFETs also leads to higher voltage stress across the SiC MOSFETs of the traditional 3L-ANPC converters due to a commutation loop inductance. Typically, in the traditional 3L-ANPC converters that employ SiC MOSFETs, large commutation loops or both the large commutation loops and small commutation loops are formed. Formation of the large commutation loops results in an increased loop inductance in the traditional 3L-ANPC converters. This increase in the loop inductance results in an increase in the voltage stress across the SiC MOSFETs, thereby resulting in a reduced lifetime of the SiC MOSFETs in the traditional 3L-ANPC converters or a converter design with reduced efficiency.
Moreover, the SiC MOSFETs typically leverage a MOSFET body diode as an anti-parallel diode to reduce cost and space. Further, in the traditional 3L-ANPC converters, the MOSFET body diodes in some SiC MOSFETs are operated to conduct current for extended periods of time. However, such body diodes cannot conduct high current for extended periods of time due to thermal and reliability constraints, thereby leading to poor reliability of the traditional 3L-ANPC converters.
When a 3L-ANPC converter is designed to only commutate in small commutation loop, overvoltages can occur during shutdown, e.g., caused by a sudden trip event. The overvoltage during a sudden shutdown is due to commutation of current using a large commutation loop, which is not used in normal operation. The leakage inductance of the large commutation loop includes not only the leakage inductance of the module, but also the external busbar. Therefore, the voltage stress at the device will be much higher when a large commutation loop is used. The modulator is designed to only use small commutation loops so that the switching speed can be increased for high efficiency, and at the same time to avoid stressing the power electronic devices. Nevertheless, when all devices are turned off at the same time during a trip event, it is possible that a large commutation loop is used. Assuming the turn-off time of a SiC MOSFET is shorter than a Si IGBT, there are multiple scenarios where the converter can potentially be damaged by overvoltage.